1. Field of the Invention
The present invention relates to an anti-skid brake system of a vehicle which suppresses an excessive braking force acting on vehicle wheels to accomplish an efficient braking action and, in particular, to a pressure reduction control just after an anti-skid brake control (Hereinafter referred to as ABS control) starts.
As a vehicle brake system, there has been known an anti-skid brake system in which a wheel lock or wheel skid is prevented under a braking condition of the vehicle. The anti-skid brake system is provided with wheel speed sensors for sensing rotation speeds of four wheels of the vehicle, respectively, magnetic control valves for controlling a brake oil pressure, controller for controlling the operation of the control valves. The controller calculates an acceleration or deceleration of the wheels based on the wheel speeds detected and reduces a brake pressure of hydraulic fluid in the control system by controlling the magnetic valves when the acceleration or deceleration of the wheel rotation is lower than a predetermined value. The controller also increases the brake pressure by controlling the magnetic valves when the wheel rotation is restored as a result of the brake pressure reduction according to the anti-skid control and when the acceleration or the deceleration is increased beyond a predetermined value. The above anti-skid brake control includes a pressure increase phase, increased pressure hold phase, pressure reduction phase and a reduced pressure hold phase in a control cycle, or may simply include the pressure increase phase and pressure reduction phase in one control cycle. The ABS control is made to cyclically execute the above phases until the vehicle is stopped. The ABS control is advantageous in that the wheel lock or wheel skid can be effectively suppressed even under an abrupt braking action to stop the vehicle with a short braking distance and with stable steering.
Japanese Patent publication 50-32709 discloses a device which reduces the brake pressure when Vw=&lt;S*V+k*WG wherein the wheel speed is Vw, the vehicle speed is V, the wheel deceleration is WG, the slip ratio (Vw/V) is S, and a predetermined constant is k.
Generally, the brake pressure reduction is made in a manner of repeating a plurality of intermittent pressure reduction actions each lasting several milliseconds. The amount of the pressure reduction of each action is determined based on the slip ratio and the wheel deceleration. Thus, the pressure reduction amount in the first control cycle in the ABS control as well as the second or after control cycle is determined based on the slip ratio and the wheel deceleration. It should, however, be noted that the rotation of the wheel is not stable in a condition just after the ABS control is started. Consequently, the wheel speed and/or the wheel deceleration would be changed greatly. Therefore, if the pressure reduction amount is determined based on the wheel speed and the wheel deceleration, the reduction amount would be too much or too small. Particularly, where the frictional coefficient .mu. of the road is small, the pressure reduction amount tends to be too much to cause wheel lock.